For measuring the illuminating power of light it is already known to couple the light into an optical fibre and connect a radiometer at the other end of the fibre. The advantage of such an arrangement is that it is relatively simple and inexpensive. However, there are a number of sources of error, which make the measuring method inaccurate and the arrangement difficult to calibrate. The light which is transmitted to the radiometer is namely affected by fibre attenuation and contact attenuation. Furthermore the light is attenuated spectrally in the fibre in an uncontrollable manner.
A still further conventional measuring method is described in the article "Photocurrent-to-frequency converter notes light levels", published in Electronics, Feb. 10, 1982 on pages 141-143. According to this method the light that is to have its illuminating power measured is converted to a frequency depending on the illuminating power. The conversion is performed with the aid of a photo diode which generates an electric current in response to the illuminating power. The current charges a capacitor, after which a level-sensing circuit generates a voltage pulse when the capacitor voltage has attained a given value. The capacitor is discharged simultaneously, after which the cycle is repeated at a rate which is dependent on the mentioned current, and thereby on the incident illuminating power. A disadvantage with this method is that an outside voltage must be supplied to the measuring apparatus for supplying the components included therein.